Its geography varies across its immense area. Within the contential U.S., eight distinct physiographic divisions exist, though each is composed of yet small physiographic subdivisions. These major divisions are the:
The Atlantic coast of the United States is, with minor exceptions, low. Here, lie the Atlantic Plain and Appalachian Highlands. The Appalachian Highland owes its oblique northeast-southwest trend to crustal deformations which in very early geological time gave a beginning to what later came to be the Appalachian mountain system. This system had its climax of deformation so long ago (probably in Permian time) that it has since then been very generally reduced to moderate or low relief. It owes its present day altitude either to renewed elevations along the earlier lines or to the survival of the most resistant rocks as residual mountains. The oblique trend of this coast would be even more pronounced but for a comparatively modern crustal movement, causing a depression in the northeast resulting in an encroachment of the sea upon the land. Additionally, the southeastern section has undergone an elevation resulting in the advance of the land upon the sea.
The following map below, known as a physiographical map, shows geographical and topographical information about the regions of the contiguous 48 states of the U.S. used by earth scientists. The map indicates the age of the exposed surface as well as the type of terrain. More information about the regions is covered in several subarticles found in the additional topics subsection below.
|Physiographic Regions of the United States|
While the east coast is relatively low, the Pacific coast is, with few exceptions, hilly or mountainous. This coast has been defined chiefly by geologically recent crustal deformations, and hence still preserves a greater relief than that of the Atlantic.
The low Atlantic coast and the hilly or mountainous Pacific coast foreshadow the leading features in the distribution of mountains within the United States. The east coast Appalachian system, originally forest covered, is relatively low and narrow and is bordered on the southeast and south by an important coastal plain. The Cordilleran System[?] on the western side of the continent is lofty, broad and complicated having two branches, the Rocky Mountain System and the Pacific Mountain System. In between these, lie the Intermontaine Plateaus. Heavy forests cover the northwest coast, but elsewhere trees are found only on the higher ranges below the Alpine region. The intermontane valleys, plateaus and basins range from treeless to desert with the very arid region being in the southwest.
The numerous rivers that drain the Atlantic slope of the Appalachians are comparatively short while those that drain the Pacific slope include only three:
Both the Columbia and Colorado rise far inland near the easternmost members of the Cordilleran system, and flow through plateaus and intermont basins to the ocean.
The Laurentian Highlands, the Interior Plains and the Interior Highlands lie between the two coasts, stretching from the Gulf of Mexico northward, far beyond the national boundary, to the Arctic Ocean. The central plains are divided by a hardly perceptible height of land into a Canadian and a United States portion. It is from the United States side, that the great Mississippi system discharges southward to the Gulf of Mexico. The upper Mississippi and some of the Ohio basin is the semi-arid prairie region, with trees originally only along the watercourses. The uplands towards the Appalachians were included in the great eastern forested area, while the western part of the plains has so dry a climate that its native plant life is scanty, and in the south it is practically barren.
The continental U.S. is often subdivided into 6 major cultural regions which not so coincidentally often share common natural features and terrain as well as similar ethnic groups. Those regions are:
Climate: mostly temperate, but tropical in Hawaii and Florida, arctic in Alaska, semiarid in the Great Plains west of the Mississippi River, and arid in the Great Basin of the southwest; low winter temperatures in the northwest are ameliorated occasionally in January and February by warm chinook winds[?] from the eastern slopes of the Rocky Mountains
Terrain: vast central plain, mountains in west, hills and low mountains in east; rugged mountains and broad river valleys in Alaska; rugged, volcanic topography in Hawaii
Land use: arable land: 19% permanent crops: 0% permanent pastures: 25% forests and woodland: 30% other: 26% (1993 est.)
Irrigated land: 207,000 sq km (1993 est.)
Natural hazards: tsunamis, volcanoes, and earthquake activity around Pacific Basin; hurricanes along the Atlantic and Gulf of Mexico coasts; tornadoes in the midwest and southeast; mud slides in California; forest fires in the west; flooding; permafrost in northern Alaska, a major impediment to development
Environment - current issues: air pollution resulting in acid rain in both the US and Canada; the US is the largest single emitter of carbon dioxide from the burning of fossil fuels; water pollution from runoff of pesticides and fertilizers; very limited natural fresh water resources in much of the western part of the country require careful management; desertification
Environment - international agreements: party to: Air Pollution, Air Pollution-Nitrogen Oxides, Antarctic-Environmental Protocol, Antarctic Treaty, Climate Change, Endangered Species, Environmental Modification, Marine Dumping, Marine Life Conservation, Nuclear Test Ban, Ozone Layer Protection, Ship Pollution, Tropical Timber 83, Tropical Timber 94, Wetlands, Whaling signed, but not ratified: Air Pollution-Persistent Organic Pollutants, Air Pollution-Volatile Organic Compounds, Biodiversity, Climate Change-Kyoto Protocol, Desertification, Hazardous Wastes